Important considerations in the selection of position sensitive and intensity detectors include high efficiency, coordinate precision, time response and background conditions. Other important considerations include operating conditions, stability, compatibility with high electromagnetic and radiation field environments, physical size, and material budget and fabrication cost.
Silicon micro strip technology is principal detector technology employed in the area of semiconductor intensity and coordinate detection of ionizing particles. Silicon micro strip technology is based on a method of detecting ionization losses in semiconductors by situating an array of the strip electrodes on a substrate and collecting the charge created during the process of ionization, when charged particles cross the sensitive volume of the strip detector. A disadvantage of this technology resides in the relatively thick sensitive area (e.g., 300 to 400 microns) needed to provide a signal high enough to detect the particles over noise. The sensitive area thickness significantly increases the material budget and decreases coordinate measurement precision. Special semiconductor material may be selected to reduce the depletion area thickness; however, the material is expensive.
Monolithic active pixel sensor technology also has been employed in the area of intensity and coordinate detection of ionizing particles. “A monolithic active pixel sensor for charged particles and imaging using standard VLSI CMOS technology,” Nuclear Instruments and Methods in Physics Research, A458, 2001. Active pixel sensors are generally made using standard VLSI technology, usually CMOS. Each active pixel sensor includes a sensing element formed in a semiconductor substrate and amplifier integrated in the sensors and capable of converting ionization losses into electronic signals. Once collected the charge carriers are transferred to output circuitry for processing. The active pixel sensor technology may be employed to provide two dimensional coordinate detectors. Disadvantages of the technology include low signal to noise ratio, poor efficiency, and complexity, such as the placement of analog electronics on the same substrate with sensitive elements.
The publication of Golovin and Saveliev, entitled “Novel type of avalanche photodetector with Geiger mode operation,” Nuclear Instruments and Methods in Physics Research” 518 (2004) 560-564 discloses an avalanche photosensor with a structure including multiple Geiger mode operation cells with a quenching mechanism (resistive layer) and a common electrode. The structure is also referred to as a silicon photomultiplier. An absorbed photon entering the micro-cell generates an electron-hole pair. Due to a high electric field inside the micro-cell, a drifting electron can generate a large number of electron-hole pairs via an avalanche process, resulting in break down of the pn junction of the micro-cell. The resistive layer covers the avalanche structure of the micro-cell for the purpose of quenching the avalanche process in the micro-cell.
The physics of semiconductor avalanche photo detectors such as silicon photomultipliers permits the detection of ionizing particles passing through the sensitive volume due to ionization processes inside the sensitive volume. When the charged particle passes the sensitive area of a silicon photomultiplier, it creates an electron hole pair due to ionization process, which initiates the avalanche process of amplification and gives a high signal to noise ratio even at the room temperature.
A problem arising from the use of silicon photomultipliers in the detection of the ionizing particles is that a signal produced by the breakdown-mode micro-cell is not proportional to the number of carriers created in the sensitive volume. Because of this loss of linearity, a signal produced by absorbed ionizing particles is indistinguishable from signals produced by a thermally generated electron/hole pair, known as dark rate pulses. As a result, the signal detection efficiency is reduced in both the intensity measurement mode and the coordinate detection mode.